The present invention relates to optical display systems. More specifically, the present invention relates to an optical display system including a light valve.
High quality, multi-media computer-based presentations are becoming commonplace at meetings and conferences. Among the reasons the computer-based presentations are becoming more commonplace is the increasing affordability and quality of the projection systems that display the presentations.
A typical projection system includes a source of bright white light, a means for separating the white light into red, green and blue spectral components, and a light valve for spatially modulating each of the components. The light valve performs the spatial modulation in response to a video signal generated by the computer. Produced by the light valve are images in red, green and blue color planes. The color planes may then be recombined and projected onto a screen, or they may be flashed in sequence onto the screen at such a rate that a viewer perceives only a single image.
There are different types of light valves. For instance, there are transmissive liquid crystal light valves and reflective liquid crystal light valves. In a transmissive light valve, viewable light enters one side and exits an opposite side. In a reflective light valve, viewable light is focused onto and reflected at the same side.
Transmissive light valves are typically fabricated on glass substrates, whereas reflective light valves are typically fabricated on silicon. Silicon semiconductor technology is more established and more readily available than semiconductor-on-glass technology; therefore, the reflective light valves are less expensive to fabricate than the transmissive light valves. Finer structures such as electrodes and address lines can be fabricated on silicon; therefore, the reflective light valves can be made smaller than transmissive light valves. Reflective light valves have higher through puts and faster switching speeds than transmissive light valves. Additionally, address lines and pixel storage capacitors do not block light in reflective light valves.
There are certain problems associated with projection systems based on light valves, problems that can be attributed to the light source. A typical light source includes a reflector and an arc lamp that extends through a hole in the reflector (for example, see the amp 12 in FIG. 1). The arc lamp generates white light, and the reflector collects light rays of various cone half-angles and focuses the light into a beam.
Because of the hole, however, the reflector does not collect and reflect light rays having small cone half-angles. Only larger cone half-angles are collected and reflected. An exemplary cone half-angle distribution is indicated by the solid line A shown in FIG. 3. The distribution indicated by the line A indicates a lack of rays having cone half-angles between +10 degrees and -10 degrees. Resulting is a dark spot in the reflected light. Darks spots in the reflected light can cause dark spots in the displayed image.
Additionally, there are "hot spots" (that is, non-uniform intensity distribution) in the beam produced by the reflector. The hot spots can also cause dark spots in the displayed image.
There are other problems associated with arc lamps. Light generated by an arc lamp might have partial coherence. The partial coherence leads to a problem known as speckle, which can cause a graininess in the projected image.
Arc lamps also have a relatively short lifetime. They might burn out at inconvenient times (e.g., during presentations) and they are replaced frequently.
There exists a need to overcome the problems associated with arc lamps. Simply replacing the arc lamp with a laser will not offer a satisfactory solution because of the full coherence in the laser light. Speckle would be a much greater problem with lasers than with arc lamps.